Internet Engineering Task Force                              A. Malhotra
Internet-Draft                                               S. Goldberg
Intended status: Standards Track                       Boston University
Expires: May 27, 2017                                  November 23, 2016


       Message Authentication Code for the Network Time Protocol
                         draft-ietf-ntp-mac-00

Abstract

   RFC 5905 [RFC5905] states that Network Time Protocol (NTP) packets
   should be authenticated by appending a 128-bit key to the NTP data,
   and hashing the result with MD5 to obtain a 128-bit tag.  This
   document deprecates the authentication of NTP packets with MD5 and
   recommends the use of AES-CMAC [RFC4493] as a replacement.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Deprecating MD5 . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Replacement Recommendation  . . . . . . . . . . . . . . . . .   2
   4.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   3
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   3
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   RFC 5905 [RFC5905] states that Network Time Protocol (NTP) packets
   should be authenticated by appending a 128-bit key to the NTP data,
   and hashing the result with MD5 to obtain a 128-bit tag.  This
   document deprecates the authentication of NTP packets with MD5 and
   recommends the use of AES-CMAC RFC 4493 [RFC4493] as a replacement.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Deprecating MD5

   RFC 5905 [RFC5905] defines how the MD5 digest algorithm in RFC 1321
   [RFC1321] can be used as a message authentication code (MAC) for
   authenticating NTP packets.  However, as discussed in [BCK] and RFC
   6151 [RFC6151], this is not a secure MAC and therefore MUST be
   deprecated.

3.  Replacement Recommendation

   If authentication is implemented, then AES-CMAC as specified in RFC
   4493 [RFC4493] should be computed over all fields in the NTP header,
   as well as the NTP key-id field, and any NTP extension fields that
   are present in the NTP packet.  We recommend that the MAC key for NTP
   should be 128 bits long AES-128 key and the resulting MAC tag should
   be 128 bits long as stated in section 2.4 of RFC 4493 [RFC4493].








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4.  Motivation

   AES-CMAC is recommended for the following reasons:

   1.  It is an IETF standard that is available in many open source
       implementations.

   2.  It is immune to nonce-reuse vulnerabilities (e.g.  [Joux])
       because it does not use a nonce.

   3.  It has fine performance in terms of latency and throughput.
       These are important considerations for NTP, since latency
       directly affects jitter and therefore the accuracy of time
       synchronization.

   4.  It benefits from native hardware support, for instance, Intel's
       New Instruction set.

5.  Acknowledgements

   The authors wish to acknowledge useful discussions with Leen
   Alshenibr, Daniel Franke, Ethan Heilman, Kenny Paterson, Leonid
   Reyzin, Harlan Stenn, and Mayank Varia.

6.  References

6.1.  Normative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <http://www.rfc-editor.org/info/rfc1321>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4493]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
              2006, <http://www.rfc-editor.org/info/rfc4493>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.






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   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <http://www.rfc-editor.org/info/rfc6151>.

6.2.  Informative References

   [BCK]      Bellare, M., Canetti, R., and H. Krawczyk, "Keyed Hash
              Functions and Message Authentication", in Proceedings of
              Crypto'96, 1996.

   [Joux]     Joux, A., "Authentication Failures in NIST version of
              GCM",
              <http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/
              comments/800-38_Series-Drafts/GCM/Joux_comments.pdf>.

Authors' Addresses

   Aanchal Malhotra
   Boston University
   111 Cummington St
   Boston, MA  02215
   US

   Email: aanchal4@bu.edu


   Sharon Goldberg
   Boston University
   111 Cummington St
   Boston, MA  02215
   US

   Email: goldbe@cs.bu.edu

















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